In extractive distillation, a nonvolatile polar solvent is added to the EDC to increase the relative volatility between the polar and less-polar components that have close-boiling points. Typically, the solvent is added to the upper portion of the EDC and the hydrocarbon feed is introduced into the middle portion of the EDC. As the nonvolatile solvent descends through the column, it preferentially extracts the polar components to form a rich solvent that moves toward the bottom of the EDC and allows the less-polar component vapors to ascend to the top of the column. The overhead vapor is condensed and a portion of the condensate is recycled to the top of the EDC as reflux while the remaining portion is withdrawn as raffinate product. The rich solvent stream containing the solvent and the polar components is fed to a solvent recovery column (SRC) to recover the polar components as an overhead product and the lean solvent (free of the feed components) as a bottom product, which is recycled to the upper portion of the EDC to be reused as extractive solvent. A portion of the overhead product is recycled to the top of the SRC as the reflux to knock down any entrained solvent in the overhead vapor. The SRC is optionally operated under reduced pressure (vacuum) and/or with a stripping medium to reduce the column bottom temperature.
ED processes to separate aromatic and non-aromatic components are described in U.S. Pat. No. 7,078,580 to Tian, et al., U.S. Pat. No. 4,053,369 to Cines, and F. Lee, et al., “Two Liquid-Phase Extractive Distillation for Aromatics Recovery”, Ind. Eng. Chem. Res. (26) No. 3, 564-573, 1987. ED techniques to separate diolefin and olefin components are described in U.S. Pat. No. 4,269,668 to Patel and to separate cycloparaffins and paraffins are described in R. Brown, et al., “Way To Purify Cyclohexane”, Hydrocarbon Processing, 83-86, May 1991. Finally, ED processes to separate styrene and C8 aromatics are describes in U.S. Pat. No. 5,849,982 to Lee, et al.
Recovering aromatic hydrocarbons from mixtures containing aromatic and non-aromatic hydrocarbons can be achieved through liquid-liquid extraction (LLE) or ED. ED processes require less equipment than LLE processes, for example, ED typically requires two separation columns as compared to four for LLE. Moreover, ED requires less energy and encounters fewer operational problems; however, application of the ED process is constrained by the boiling range of the feedstock. Thus, in order for the ED process to achieve acceptable levels of aromatic purity and recovery, the solvent must retain essentially all the benzene, which is the lightest aromatic compound with a boiling point (BP) of 80.1° C., in the EDC bottom; in addition, the process must drive virtually all of the heaviest non-aromatics into the overhead of the EDC. For a narrow boiling-range (C6-C7) aromatic feedstock, the heaviest non-aromatic compound can be ethylcyclopentane (BP: 103.5° C.) whereas for the full boiling-range (C6-C8) aromatic feedstock, the heaviest non-aromatic compound can be ethylcyclohexane (BP: 131.8° C.). As is apparent, it is much more difficult to recover benzene, toluene, and xylene (BTX) aromatics from a full boiling-range feedstock, such as a full range pyrolysis gasoline, than it is to recover benzene and toluene from a narrow boiling-range feedstock, such as C6-C7 reformate. An ED process, that is suitable for the narrow boiling-range aromatic feedstock, may not be able to satisfactorily process the full boiling-range aromatic feedstock.
Another critical problem associated with the ED process for aromatics recovery is the existence of measurable amounts of heavy (C9-C12) hydrocarbons in the ED feedstock, which in severe situations may cause the process to fail. This problem is of special concern in the recovering of BTX aromatics from the full boiling-range (C6-C8) feedstock. For aromatics recovery in ED and LLE processes, the solvent is circulated indefinitely in a closed loop configuration. To remove the heavy hydrocarbons and the polymerized heavy materials derivates from oxidized solvent, commercial LLE processes use a solvent regenerator whereby a small slip stream of the lean solvent (approximately 1% of the lean solvent stream) is heated with or without stripping steam to recover regenerated solvent and/or any heavy components having boiling points lower than that of the solvent. The heavy polymeric materials having boiling points higher than that of the solvent are removed from the bottom of the solvent regenerator as sludge.
The solvent regeneration scheme disclosed in U.S. Pat. No. 4,048,062 to Asselin was successfully implemented in UOP and IFP LLE aromatics recovery processes using sulfolane and water as the extractive solvent. The reason for the success was that most of the heavy (C9 to C12) hydrocarbons in the feedstock were rejected by the solvent phase in the LLE column and were removed along with the raffinate phase as part of the non-aromatic product. Only a portion of C9 aromatic compounds were subject to extraction by the solvent and they can be effectively stripped from the solvent in the SRC under normal operating conditions.
In normal EDC operations for aromatics recovery, however, these heavy hydrocarbons tend to remain in the rich solvent at the bottom of the EDC due to their high boiling points. For the full boiling-range (C6-C8) feed, the high boiling points of the heavy hydrocarbons prevent them from being stripped from the solvent in the SRC so these heavy hydrocarbons accumulate as the solvent is circulated indefinitely in a closed loop between the EDC and the SRC. The solvent regeneration scheme that is described in U.S. Pat. No. 4,048,062 is not applicable for ED process since it is designed for LLE processes for removing minor amounts of polymeric materials that are formed possibly from reactions between the oxidized or decomposed solvent components and traces of heavy hydrocarbons in the solvent. The industry is in need of methods to adequately remove heavy hydrocarbons and/or polymers from the lean solvent of the ED process for recovering aromatics.
C4 hydrocarbons mixtures containing C4 of different degrees of unsaturation, such as mixtures of butanes and butenes and mixtures of 1,3 butadiene with butanes and butenes, are not easily separated by ordinary fractional distillation because of the similarities in boiling points of the constituents and the formation of azeotropes. However, these mixtures are much more efficiently separated into their individual components by an ED process using a water-soluble solvent of relatively higher boiling point which selectively dissolves one or more of the more-unsaturated components. The commercially practiced solvents include furfural, acetonitrile, dimethyl formamide, dimethyl acetamide, and N-methylpyrrolidone, 3-methoxy propionitrile, and their mixtures with water. U.S. Pat. Nos. 3,309,412 and 3,551,507 both to Sakuragi et al. recognized the polymerization of butadiene in the ED process and taught adding minor amounts of inhibitors, such as furfural, benzaldehyde, nitrophenol, or dinitrophenol, to the solvents in order to minimize this problem. The inhibitors reduce the level of polymerization but accumulated polymers must still be removed from the lean solvent.
U.S. Pat. No. 5,849,982 to Lee et al. disclosed an ED process, for recovering styrene from the C8 fraction of pyrolysis gasoline, which uses water-soluble solvents including propylene carbonate, sulfolane, methyl carbitoal, 1-methyl-2-pyrrolidone, 2-pyrrolidone and their mixtures with water. No polymer inhibitor was added in this process even though styrene has a tendency to form polymers under thermal conditions. The lean solvent was regenerated with a conventional solvent regenerator using energy derived from steam stripping and reboiler heat. The ED process might not remove all the generated polymers.
Current extractive solvents for liquid-liquid extraction or extractive distillation are water-soluble, especially sulfolane, polyalkylene glycols, N-substituted morpholine, furfural, acetonitrile, dimethyl formamide, dimethyl acetamide, N-methylpyrrolidone, and 3-methoxy propionitrile. Indeed, because they are water-soluble, extractive solvent can be removed in minor amounts from the raffinate stream generated in the extraction zone of the LLE process, through counter-current or co-current extraction with water, to produce a solvent-free raffinate product stream. The solvent may be present in the raffinate stream partly as an equilibrium constituent in low concentrations and partly as an entrained dispersion of free solvent phase due to the turbulence within the extraction zone. Aromatic purification via LLE processes is further described in U.S. Pat. No. 4,419,226 to Asselin wherein the solvent composition comprises sulfolane and water and in U.S. Pat. No. 2,773,918 to Stephens wherein the solvent comprises polyalkylene glycol and water. Both techniques include the step of extracting a water-soluble solvent with water from a non-aromatic raffinate stream exiting the extraction zone of the LLE process. The water-soluble property of the solvent can also be used to maintain the concentration of the co-solvent in the solvent mixture as disclosed in U.S. Pat. No. 6,551,502 to Lee, et al.
In an ordinary distillation column, the overhead liquid reflux is essential to generate the liquid phase, in the rectifying section of the column, which contacts the uprising vapor phase from tray-to-tray for separating the key components in the feed mixture. Depending on the application, the normal reflux-to-distillate ratio in a distillation column is approximately 1 to 20. In the EDC, however, the liquid phase in the rectifying section is the nonvolatile, polar solvent, which preferentially absorbs the more-polar components from the uprising vapor phase and allows the less-polar component vapors to ascend to the top of the EDC. The present invention recognizes that adding reflux to the EDC may not significantly enhance purity and recovery of the EDC overhead product (the raffinate). Its sole purpose is to knock down the entrained solvent in the raffinate product. In fact, elimination of the EDC reflux can substantially reduce the steam consumption in the bottom reboiler and reduce the vapor loading of the upper portion of the column, thereby, increasing the column throughput.
Finally, two liquid phases may occur in the EDC if the extractive solvent has very limited solubility of the less polar components in the feed mixture. For example, an ED process for purifying the aromatics using sulfolane containing water as the solvent was disclosed by U.S. Pat. No. 4,053,369 to Cines, where two liquid phases existed in the upper portion of the EDC. This is due to the fact that sulfolane has very limited solubility with the less polar non-aromatics, which are concentrated in the upper part of the EDC. However, contrary to Cines' teaching, the present invention further recognizes that the existence of two liquid phases in an EDC is quite undesirable and may create many potential operating problems in the EDC. Limited experimental data available in the literature suggest that the column (or tray) efficiencies are quite low and highly variable in the range of 25 to 50% in the two-liquid phase region. Adding reflux to the EDC causes expansion of the two-liquid phase region in the upper portion of the EDC since the reflux is essentially 100% raffinate, which is less soluble in the solvent. Without EDC reflux, different methods must be developed to eliminate the entrained solvent in the raffinate product from the EDC overhead stream.